Typical exhaust conversion catalysts contain small amounts of Group VIII metals, particularly platinum, palladium, and rhodium, dispersed on a high surface area inert support material such as alumina. While gamma alumina is often preferred as the support, other forms of alumina such as delta, eta, theta, and alpha alumina may be present. Other materials such as zirconia, titania, silica, silica-alumina, and alumina-silicates have also been suggested in the art as supports.
For optimum performance exhaust conversion catalysts will often additionally contain promoters, such as alkaline earth oxides including barium and strontium, transition metals including iron and nickel, and rare earth oxides including cerium and lanthanum. The function of these promoters is not fully understood, but it is thought that they have several functions, including stabilization of the supports to prevent or limit phase changes and loss of surface area.
Motor vehicle exhaust conversion catalysts are normally operated under conditions which inherently swing between oxidizing and reducing as an oxygen sensor and its control system keep the air/fuel ratio within the desired operating A/F window around the stoichiometric value. Ceria is a well-known component of such exhaust conversion catalysts. It is often referred to as an "oxygen storage" agent because it is considered to have the ability to give up oxygen when the catalyst is exposed to reducing conditions and to re-oxidize when exposed to oxidizing conditions. It has also been suggested that ceria may stabilize the support structure, promote the activity of the precious metals, or promote the water gas shift reaction. See for example, B. Harrison, A. F. Diwell and C. Hallet, Platinum Metals Rev., 1988, 32(2), 73-78.
In their report on the deactivation of catalysts, Funabiki and Yamada, SAE Technical Paper Services 881684, studied the increase in platinum and ceria crystallite size as operating temperature was increased. They concluded that smaller ceria crystallites provide greater oxygen adsorption and better conversion of CO, HC, and NO.sub.x. However, it appears that the smallest ceria crystallites measured were about 200 .ANG..
In U.S. Pat. No. 4,791,091 Bricker disclosed an auto exhaust catalyst having lanthanum crystallites below 25 .ANG. size which are produced by a unique process combining hydrosols of alumina with solutions of lanthanum compounds.
In U.S. Pat. No. 4,868,149 Bricker et al. disclosed another catalyst in which lanthanum is present in crystallites below 25 .ANG., but the lanthanum was deposited by impregnating a washcoated monolith with lanthanum nitrate. The crystallite size of ceria in such catalysts was reported to ie 90-100 .ANG. where ceria was deposited on the alumina washcoat by impregnation.
One method of applying Group VIII metals to a catalyst support is to employ an impregnating solution including hydroxycarboxylic acid in order to cause the dissolved Group VIII metal compound to penetrate into the support. One example is found in Japanese published application J57119838 assigned to Mitsui Mining & Smelting. They disclose the impregnation of a washcoated ceramic honeycomb with an aqueous solution of palladium chloride and cerium acetate including 150-330 gm/liter of a hydroxylcarboxylic acid.
Ceria promoted exhaust conversion catalysts have become a major type of exhaust conversion catalyst and the recent tend has been to prepare catalysts with increased Ce loadings to achieve higher catalyst performance. However, the amount of Ce in these catalysts has become an issue as the cost of Ce is increasing rapidly (approx. 10%/year). Thus, it would be an advantage in the art to be able to reduce the amount of metal oxide required to produce effective promoted supports for exhaust conversion catalysts.
One way to reduce the Ce loading and/or increase the effectiveness of a lower Ce loading is to increase the dispersion of Ce. In U.S. Pat. No. 5,064,803, there is disclosed a method for preparing more highly dispersed Ce catalysts for conversion of the exhaust from internal combustion engines that includes impregnating a support with a solution of a cerium compound and an organic compound containing hydroxy and/or carboxylic acid moieties. The catalyst resulting from the method showed crystallite sizes of around 50 .ANG. as measured by X-ray diffraction.
Another way to reduce the amount of Ce required is to promote the effectiveness of the Ce employed by the inclusion of a secondary promoter in the ceria crystalline lattice.